One of the main sources of fuel for the engines of rockets and missiles is propellant fuel which is in the form of a liquid. This liquid propellant is typically stored in cylindrical storage tanks located within the body of the rocket or missile. A movable piston is slidably mounted within the storage tank and is used to maintain the pressure of the liquid fuel in the storage chamber of the tank and to forcibly expel the liquid propellant from the tank through an appropriately located orifice under adverse acceleration environments when desired. Another fluid (a liquid, a gas, a gel, a suspension of solids, or a mixture of any one or more of these) is typically used as a pressurant to cause the piston to slide within the tank to pressurize the propellant or, alternatively, to expel the liquid fuel from the tank.
Since the liquid propellant may have to remain in its storage tank for many years before it is used, it is important that the liquid propellant remain within the storage tank and not seep past the piston. Such seepage of liquid propellant from the storage chamber of the rank into other portions of the rocket not only results in propellant being lost and therefore reducing the range of the rocket, but may also result in a risk of premature ignition of the rocket or in damage to other parts of the rocket, as the propellant is typically extremely caustic. Additionally, elastomeric seals are often necessary to provide a dynamic seal for the piston as it moves within the storage tank. However, the materials from which such elastomeric seals are typically made generally degrade when exposed for a substantial length of time to the liquid propellant, thereby jeopardizing proper operation of the piston and the rocket.
One solution to this problem is proposed in U.S. Pat. No. 5,042,365 wherein an annular shear seal is mounted within the storage tank about the outer circumference of the piston so that it also contacts the inner surface of the wall of the storage tank. The annular shear seal is structurally sealed, such as by welding or other bonding means, along its central opening to the outside surface of the piston and along its outer circumference to the inner surface of the wall of the storage tank. The annular shear seal is disclosed as having a notch portion which is positioned on the shear seal between the point where the shear seal contacts the piston and the point where the shear seal contacts the storage tank wall. The shear seal and its two welds block leakage of the propellant before the rocket is activated. When the rocket is to be activated, fluid pressure is applied to the piston, which causes the piston to exert an increasing force to the shear seal until a point when the shear seal ruptures along its notch. When the sear seal ruptures, the piston is free to slide and thereby expel the liquid fuel from the storage chamber of the tank. Although the disclosed shear seal design aids in preventing leakage of the propellant, the design requires at least two separate welds--one between the piston and the shear seal, and the other between the shear seal and the wall of the storage tank. Since the integrity of these two welds is critical to the proper operation of the storage tank, both of the welds must be inspected prior to final assembly of the tanks. The inspection of these two welds is a relatively time consuming and expensive process in view of the fact that they are located within the narrow confines of the storage rank and must be carried out by using X-ray photography.
Additionally, a shear seal typically does not rupture uniformly along its the entire length of its notch due to variations of the notch profile, variations in the depth of the notch, variations in the thickness of the surrounding material, and variations in the metal comprising the shear seal itself. As a result, it may be expected that a notched shear seal as disclosed above will not break uniformly, at exactly the same position, and at the same moment along its entire length. Furthermore, for the elastomeric seals to operate properly (i.e., provide a fluid tight seal while not impinging upon the freedom of movement of the piston), the rupture point of a shear seal cannot be recessed too deeply into the wall of the storage tank because the elastomeric seals will have to pass over the recess in which the shear seal is mounted. As a result, jagged remnants of the shear seal may be expected to remain, protrude, and tear or score the elastomeric annular seals on the piston. Such tears or scoring of the elastomeric seals allow the propellant to leak around the piston when the piston is activated, thereby possibly causing the piston to malfunction. Remnants of the shear seal may also cause the piston to jam so that all of the propellant cannot be expelled.
U.S. Pat. No. 3,545.343 discloses a design which employs only one weld rather than two or more welds, however, this weld is very deep. In this design, a weld is applied from the outside of the storage tank and passes through the entire wall of the storage tank into a recess in the piston. The weld is disclosed to be thin enough so that it will rupture or shear when the piston begins to move. This design poses problems relating to the formation of a uniform deep weld, and to the evaluation of the integrity of the weld because of its depth. In addition, the design does not protect the elastomeric seals on the piston from getting torn or scored by remnants of the weld after the weld has ruptured.